Macrocyclic lactone resistance in nematodes of cattle in Brazil: Blame it to the ticks!

Linalyl acetate against larvae of Haemonchus spp. and Trichostrongylus spp. that affects ruminants: considerations about the hormetic effect

Active components from plants are an alternative therapy to parasite control, addressing the widespread multidrug resistance populations. Linalyl acetate (LA), an ester abundantly found in plants of the genus Lavandula, was tested in vitro against third-stage larvae (L3) of Haemonchus spp. and Trichostrongylus spp. using the larval migration test at 0.89, 2.24, 4.47, 8.95, 17.9, 35.8, 71.6, and 143.2 mg/ml. After an initial incubation of 18 h, the total content of each tube was transferred to a 24-well plate that allowed active L3 to migrate through a nylon mesh (second incubation). Although LA exhibited 100% efficacy in reducing larval migration at 8.95 and 17.9 mg/ml, it showed reduced activity (5%) at 143.2 mg/ml. The data revealed a hormetic biphasic response characterised by an inverted U-shaped concentration-response curve. While hormesis has been previously documented in insecticidal and allelopathic contexts, this study reports the occurrence of hormesis induced by a phytochemical component against two species of nematodes for the first time. This distinctive stimulation-and-inhibition effect should be considered when selecting new compounds for preclinical testing.

Anthelmintic resistance of gastrointestinal nematodes in cattle in Brazil and Argentina - current status and global perspectives

This review outlines the current state of anthelmintic resistance (AHR) of gastrointestinal nematodes (GINs) among cattle in Argentina and Brazil, emphasizing the economic repercussions, animal health and welfare. The analysis explores factors associated with AHR and proposes a potential solution: the use of drug combinations. Both countries are grappling with a severe AHR scenario in cattle, having progressed through incipient, established, and advanced phases, leading to extreme cases of animal mortality due to ineffective control strategies. Genera such as Cooperia and Haemonchus have the highest reports of resistance, with Oesophagostomum radiatum also posing significant problems. While oral benzimidazoles and levamisole remain effective in most herds, moxidectin is entering an advanced resistance phase, and avermectins are increasingly deemed ineffective. The review explores the impact ofclimate, mixed grazing, animal movement and other husbandry practices, and the relationship between ectoparasite control and the emergence of resistant helminths. Notably, the discussion includes the strategic use of drug combinations as a valuable approach to address resistant GINs control in livestock, highlighting its significant potential to mitigate the challenges posed by AHR in the cattle industry of these countries.

Phenotypic and genotypic characterization of acaricide resistance in Rhipicephalus microplus field isolates from South Africa and Brazil

Rhipicephalus (Boophilus) microplus is one of the most successful ticks infesting cattle around the world. This highly-invasive species transmits cattle parasites that cause cattle fever leading to a high socio-economic burden. Tick eradication programs have often failed, due to the development of acaricide resistance. Here we characterize acaricide resistance in a large number of tick isolates from regions in South Africa (KwaZulu Natal, Mpumalanga, Western & Eastern Cape provinces) and two Brazilian regions. By means of Larval Packet Tests (LPT's) acaricide resistance was evaluated against five commonly used acaricides (chlorfenvinphos, fipronil, deltamethrin, amitraz, and ivermectin). Furthermore, the coding region containing the knock down resistance (kdr) mutation, known to result in pyrethroid resistance, was sequenced. Resistance to at least one acaricide class was reported in each of the five regions, and a high proportion of tick isolates exhibited multi-resistance to at least two acaricide classes (range: 22.2-80.0%). Furthermore, resistance ratios (RR) showed high spatial variation (intercontinental, as well as regional) but low regional spatial autocorrelation. Previous and current acaricide use correlated with current RR, and several combinations of acaricide RR were positively correlated. Moreover, fipronil resistance tended to be higher in farms with more intense acaricide use. The kdr-mutations provided the ticks a fitness advantage under the selection pressure of synthetic pyrethroids based on population (kdr-allele frequency) and individual level data (genotypes). The data show the threat of acaricide (multi-)resistance is high in Brazil and South Africa, but acaricide specific levels need to be assessed locally. For this purpose, gathering complementary molecular information on mutations that underlie resistance can reduce costs and expedite necessary actions. In an era of human-caused habitat alterations, implementing molecular data-driven programs becomes essential in overcoming tick-induced socio-economic losses.